1. Field of the Invention
The present invention relates to a motor having a hemispherical dynamic pressure bearing.
2. Brief Description of Related Art
Generally, a dynamic pressure bearing comprises a supporter, and a rotating member eccentrically and rotatably supported by the supporter. Pressure is formed by lubricant fluid within a gap defined between the supporter and the rotating member so as to enhance the rotation of the rotating member.
Particularly, a hemispherical dynamic bearing is designed such that a portion thereof for supporting the rotating member is hemispherically-shaped so as to simultaneously support a radial load and a thrust load.
Accordingly, since there is no need for a plurality of bearings for supporting each load, it is possible to make the bearing compact. In addition, when using air as fluid for forming lubricant pressure for the dynamic bearing, a seal member is not required such that the bearing can be made in a more compact state. This compact bearing is suitable for use in a motor used in electronic products.
Especially, with regard to a laser beam printer, laser beams are used to perform a printing operation. That is, a scanned image is formed on a photosensitive drum, which is responsive to light, by radiating the laser beams thereon. A rotating multi-faceted mirror system is provided to move the beams at a uniform velocity such that the beams are aligned with an axial direction of the photosensitive drum.
Referring to FIG. 1, there is shown a sectional view illustrating a conventional rotating multi-faceted mirror.
As shown in FIG. 1, the rotating multi-faceted mirror system 10 comprises a multi-faceted mirror 11 for reflecting laser beams to a photoconductive drum (not shown), a motor 20 for rotatably supporting the multi-faceted mirror 11, and a cover for receiving the multi-faceted mirror 11 and the motor 20. The multi-faceted mirror reflects laser beams through a hole 12a formed in the cover 12. The motor 20 includes a stator 21 and a rotor 22 which electromagnetically cooperate with each other. The stator 21 has a stator frame 21a, and a stator coil 21b wound around the stator frame 21a. The rotor 22 has a rotor bushing 22a, and a rotor magnet 22b mounted around the rotor bushing 22a. Mounted on an upper side of the rotor bushing 22a is a multi-faceted mirror bracket 13 fixedly supporting the multi-faceted mirror such that the multi-faceted mirror 11 rotates with the rotating bushing 22a. Inserted into upper and lower portions of the rotor bushing 22a are respective upper and lower hemispherical bearings 24a and 24b. The upper and lower hemispherical bearings 24a and 24b are fixed on a supporting shaft 23 mounted on a central portion of the stator frame 21a. The upper and lower hemispherical bearings 24a and 24b are disposed such that their rounded portions are facing each other. In addition, a plurality of grooves (not shown) are formed on the outer circumference of each of the upper and lower hemispherical bearings 24a and 24b so that air can be induced between the rotor bushing 22a and the upper and lower hemispherical bearings 24a and 24b as lubricant air enhancing the rotation.
The hemispherical bearing and the rotor bushing are described in more detail with reference to FIG. 2.
As shown in FIG. 2, the rotor bushing 22a is provided at its top and bottom with upper and lower hemispherical grooves 25a and 25b, respectively and a communicating hole 26 connecting the upper and lower hemispherical grooves 25a and 25b to each other. Edges 27a and 27a' of the upper and lower hemispherical grooves 25a and 25b are chamfered, and upper and lower edges 27b and 27b, of the communicating hole 26 are also chamfered, such that the upper and lower hemispherical bearings 24a and 24b can be easily inserted into the grooves. The upper and lower hemispherical bearings 24a and 24b are respectively provided with coupling holes 29 and 291 through which the supporting shaft 23 passes. Edges 27c and 27c' of flat surfaces 30a and 30b of the upper and lower hemispherical bearings 24a and 24b are also chamfered.
In the conventional rotating multi-faceted mirror system 10 structured as described above, when electric power is not applied thereto, the lower hemispherical bearing 24b contacts the lower hemispherical groove 25b by a gravitational force applied to the rotor busing 22a, while the upper hemispherical bearing 24a is spaced away from the upper hemispherical groove 25a. In this state, if electric power is applied for rotating the rotor bushing 22a, air is induced between the rotor bushing 22a and the upper and lower hemispherical bearings 24a and 24b, thereby raising the rotor bushing 22a. That is, the rotor bushing 22a becomes minutely spaced away from the lower hemispherical bearing 24b by pressure formed therebetween and caused by an eccentric coupling between the upper and lower hemispherical bearings 24a and 24b and the rotor bushing 22a. At this point, the flat surfaces 30a and 30b of the upper and lower hemispherical bearings 24a and 24b are located in the same planes as the upper and lower ends of the rotor bushing 22a, respectively.
However, in the above described structure, because of the chamfered portions, the acting area (interface) between the hemispherical bearings and the rotor bushing is decreased, thereby reducing a rotational supporting efficiency of the hemispherical bearing. As a result, the rotation between the hemispherical bearing and the rotor bushing cannot be smoothly realized, whereby the surfaces of the interface become prematurely worn.